Fixed default zone file name containing .. in suffix.

Like example.com..zone

src/knot/conf/conf.c

@@ -295,7 +295,7 @@ static int conf_process(conf_t *conf)
 
 		// Default zone file
 		if (zone->file == NULL) {
-			zone->file = strcdup(zone->name, ".zone");
+			zone->file = strcdup(zone->name, "zone");
 			if (!zone->file) {
 				ret = KNOT_ENOMEM;
 				continue;